It has been proposed to burn particles such as artificial light-weight aggregates, powdered lime stones, powdered dolomite and so forth by means of a vertical furnace employing an upward flow of gas such as a combustion gas which forms various beds of the particles such as fluidized bed, jetting flow bed, improved jetting flow bed, swirling flow bed, loaded moving bed and the like.
The present inventors have already noticed the following facts in the burning of particles. In the burning of the artificial light-weight aggregates, the agglomeration by fusing of the particles takes place easily because the bloating temperature of the particles approximates the fusion temperature thereof. This problem is serious particularly when the burning is made by means of a rotary kiln, namely, in such a case, the heating of the particles due to the heat from the kiln wall, and the particles are gradually moved toward the kiln outlet while being agitated only insufficiently. Due to the radiant heat from the kiln wall and the insufficient agitation of the particles, the agglomeration by fusing of the particles is liable to occur, resulting not only in the unsmooth operation of the apparatus but also in a lowered thermal efficiency.
For effectively burning the light-weight aggregate by a rotary kiln, it is necessary to make the particles stay in the kiln for a comparatively long time which generally falls between 20 and 60 minutes. Partly because of this reason and partly because of the heat transfer mechanism as stated above, the particles of particle size less than 5 mm tend to be lost bloating element by oxidation or due to fusion, although comparatively coarse particles of mean particle size exceeding 5 mm can provide a specific gravity of 1.25 to 1.35 of the burned product. Thus, it is not possible to obtain a burned product having a specific gravity smaller than 1.55, with the fine particles having a mean particle size below 5 mm.
Generally, in the burning of the light-weight aggregate, it is said that a operation has to be made through a minute temperature control within the temperature error of 10.degree. C. at a high temperature region in excess of 1100.degree. C. to prevent the fusion of the burned products. It is prohibited to discharge the product to the outside of the furnace in a stacked manner or to cool the hot burned product in a stacked state, because the fusion of the burned particles is liable to occur due to a confinement of the heat.
With these knowledges, the present inventors have made various studies for the possibility of burning of particles with a gas stream by a vertical furnace which is considered to have a large rate of heat transfer and, hence, a reduced tendency of agglomeration of the burned particles.
Hitherto, various methods have been proposed for burning particles with gas flows in vertical furnace, as summarized below.
(1) Namely, Japanese Patent Publication No. 48076/1974 discloses a batch type method in which coarse particles having particle size in excess of 5 mm are supplied into a furnace and are burned while being suspended, fallen and circulated by an upward swirling flow of hot gas, and the burned product is discharged from the lower side of the furnace.
(2) Japanese Patent Laid-open No. 121807/1978 discloses a technique in which the particles are fluidized by an upward flow of gas which has passed through a rectifying plate (a perforated plate), and the burning treatment is conducted continuously with the supply of a material for adjusting the temperature in the furnace to avoid the fusion of the burned particles. The burned particles overflow the fluidized bed and are discharged continuously.
(3) A plurality of rectifying plates for upward flow of gas are disposed in the furnace to form a plurality of stages of fluidized bed, so that the particles overflow from each stage to fall onto the underlying bed while being successively subjected to a pre-heating, burning, and cooling, so that the burned product is stacked on the bottom of the furnace. The product is discharged while being cooled by the air to pre-heat the air conveyed into the furnace for burning.
(4) As disclosed in Japanese Patent Laid-open No. 68796/1979, an improved jetting bed is formed in a furnace by an upward flow of gas which has passed through a rectifying plate to effect the burning continuously while permitting the overflow of the burned particles from the bed for the discharge.
(5) The fluidized beds of the particles are formed without any assist of the perforated rectifying plate as used in the prior arts (2),(3) and (4) stated above. The thickness or density of the particles is greater in the lower stage than in the upper stage of the fluidized bed. The burned products are stacked on the bottom of the furnace and discharged from the latter. Other techniques proposed hitherto are more or less applications, improvements, or modifications of these five techniques summarized heretofore.
The present inventors have found that these conventional techniques, either solely or in combination, cannot satisfy the following requirements (a) to (c).
(a) To obtain a condition for satisfactorily avoiding the agglomeration of the burned particles.
(b) To obtain a sufficiently high thermal efficiency. In the conventional methods summarized above, the thermal efficiency is inevitably low because the gas is discharged at a high temperature substantially equal to the burning temperature.
(c) To obtain a large treating capacity of the furnace per unit volume.